As the title suggests, I'm a newbie to mushroom growing. I want to grow Agaricus bitorquis (this will be from a kit), but I have a question about possible cross breeding/contamination.

I live in New England in an old post-victorian house that has a tamped earth/victorian construct basement. It maintains the proper temps for mushroom growing, but sometimes when we heavy rains, I get some water in there. This often produces some fungi (mushrooms) of unknown type on the floor.

My question is this: Will the appearance of these unknown mushrooms contaminate my planted stock. My kit will be well above the floor.

Well, the spores of the unknown mushrooms will off course land on your A. bitorquis substrate, but since it is already colonized, nothing bad should happen. I would be more concerned by green and black molds which could also be in the cellar, those could potentially contaminate and destroy the substrate.